1. Field of the Invention
The present invention relates to an image pickup apparatus configured to make an exposure by combining an electronic front curtain shutter and a mechanical rear curtain shutter.
2. Description of the Related Art
Conventionally, single-lens reflex type cameras (digital cameras and silver-salt cameras or the like) control an exposure time through a mechanical shutter such as a focal plane shutter.
The focal plane shutter is generally made up of two curtains; a front curtain and a rear curtain, and the front curtain shades an image pickup device (a film in the case of a silver-salt camera, but hereinafter, an image pickup device will be taken as a representative example) before photographing (1, front curtain closed). After that, when a photographing button or the like is operated, a photographing operation starts, a charged spring is released and the front curtain thereby starts running first (2, front curtain running). Accompanying the running of the front curtain, exposure is sequentially started, for example, from the top end of the image pickup device. If the shutter speed is equal to or below a synchronized speed, when the front curtain finishes running, a state is realized in which the entire surface of the image pickup device is under exposure (3, front curtain finishes running) (however, when the shutter speed is faster than the synchronized speed, the rear curtain starts running before the front curtain finishes running, and so exposure is performed in a slit shape). When the front curtain starts running and an exposure time corresponding to the shutter speed elapses, the rear curtain starts running by releasing the charged spring and shading is sequentially started from the top end of the image pickup device (4, rear curtain running). When the rear curtain finishes running, the entire surface of the image pickup device is shaded (5, rear curtain finishes running) An image signal is then read from the image pickup device in the shaded state after the rear curtain finishes running.
In this way, the conventional camera controls an exposure start through running of the mechanical front curtain shutter and controls an exposure end through running of the mechanical rear curtain shutter, and thereby controls a time of exposure to the image pickup device when photographing a still image.
However, with a short-time exposure, it is difficult to perform mechanical control while maintaining high accuracy, and in an example of the focal plane shutter, it is difficult to control an exposure time shorter than 1/8000 seconds. Therefore, the order of 1/8000 seconds is an upper limit to the shutter speed of a camera adopting a focal plane shutter.
To solve such problems, a technique using an electronic shutter is proposed as a technique for allowing higher accuracy control using a high-speed shutter or realizing a higher speed shutter.
For example, Japanese Patent Application Laid-Open Publication No. 11-41523 describes a technique for an image pickup apparatus using an XY address type image pickup device such as a CMOS image pickup device that controls a charge accumulation start time of the image pickup device using an electronic front curtain shutter realized by pixel reset instead of a mechanical front curtain shutter and uses a mechanical rear curtain shutter (focal plane shutter) for controlling a charge accumulation end time. In this case, by controlling running characteristics of the electronic front curtain shutter (that is, running characteristics of sequentially resetting pixels from the top line to the bottom line) according to non-linear running characteristics of the mechanical rear curtain shutter, it is possible to perform still image photographing for a substantially uniform exposure time over the entire region of the image pickup device.
Here, with the aforementioned electronic front curtain shutter, pixels are generally reset collectively for every plurality of lines, instead of resetting pixels line by line (hereinafter, an electronic front curtain shutter realized by sequentially resetting pixels for every plurality of lines will be referred to as “block electronic front curtain” or the like for brevity). Adoption of such a reset method is intended to simplify the drive circuit or eliminate the necessity of an expensive drive clock.